Impact of Hofmeister anion type on the structural and mechanical properties of composite whey protein hydrogels

Abstract

Biopolymer hydrogels are widely employed in food products to provide desirable functional attributes, especially texture and mouthfeel. Hydrogels can be created from whey proteins, but they are too soft and brittle for certain applications. Previously, it was shown that the mechanical properties of whey protein hydrogels could be enhanced by embedding whey protein nanofibers in them. In the current study, the impact of soaking these nanofiber-filled composite hydrogels in salt solutions containing anions in different positions within the Hofmeister series was studied. In addition, the impact of hydrogel pH (1, 3, 7, or 9) on the properties of the hydrogels was investigated. The hardness and shear modulus of the hydrogels increased appreciably after they were soaked in the salt solutions. For example, at pH 7 and pH 9, the hardness of the composite hydrogels soaked in K2SO4, KCl, and KBr solutions were around 5.8-, 4.8-, and 2.3-fold higher and 7.3-, 6.0-, and 3.3-fold higher than that of the original hydrogels, respectively. The magnitude of increase in gel strength was consistent with the position of the anions in the Hofmeister series: SO42− > Cl− > Br−. Fourier transform infrared showed that the presence of Hofmeister ions increased the number density and strength of protein-protein bonds in the gel network by strengthening hydrogen bonding and hydrophobic interactions. Therefore, composite whey protein hydrogels with different structural and textural characteristics could be produced by controlling their pH values and/or by soaking them in salt solutions containing anions in different positions within the Hofmeister series.